Add ServerById() function and acceptance test.
diff --git a/interfaces.go b/interfaces.go
index de2c35b..16c0589 100644
--- a/interfaces.go
+++ b/interfaces.go
@@ -19,4 +19,5 @@
 // for your provider.
 type CloudServersProvider interface {
 	ListServers() ([]Server, error)
+	ServerById(id string) (*Server, error)
 }